Monday, October 29, 2012

Classification of Colloids

Since the colloidal solutions
consist of dispersed phase and dispersion medium, the components of these phases
have certain specific characteristics of their own. In view of these
observations, the colloidal solutions can be classified into the various types
as under;

(I). CLASSIFICATION BASED ON PHYSICALSTATE OF DISPERSED PHASE
AND DISPERSION MEDIUM

Based on the physical state of
dispersed phase and dispersion medium, there are eight types of colloidal
solutions. Either these phases may be solid, liquid or gas. Since gases gives
homogeneous mixture in all proportions, hence colloidal solution can not be
obtained in which dispersed phase and dispersion medium are gases.

S.No.

Dispersed
Phase

Dispersion
Medium

Name
of Colloidal Solution

Examples

1.

Gas

Liquid

Foam

Soap lather, whipped cream, soda water, froth on beer.

2.

Gas

Solid

Solid foam

Cake, Bread, Pumice stone lava.

3.

Liquid

Gas

Aerosol

Mist, fog, Clouds.

4.

Liquid

Liquid

Emulsion

Milk, cream, butter, oil in water

5.

Liquid

Solid

Gel.

Curd, Cheese, Jellies, boot polish.

6.

Solid

Gas

Solid Aerosol

Smoke, dust.

7.

Solid

Liquid

Sols

Paints, gold sol, inks.

8.

Solid

Solid

Solid Sols

Alloys, coloured glass, gem stones.

(II). CLASSIFICATION BASED ON ELECTRICAL CHARGE ON DISPERSION PHASE

Based on the nature of charge on the
dispersed phase, there are two types of colloidal solution;

(a). Positive Colloids

When dispersed phase in a colloidal solution
carries a positive charge, it is known as Positive Colloid.

When dispersed phase in a
colloidal solution carries a negative charge, it is known as Negative
Colloid.

Examples:
AS2S3 Sol, Cu or Ag Sol.

(III). CLASSIFICATION
BASED ON APPEARANCE OF THE COLLOIDS

On this basis, the colloidal
solutions can be divided into two types;

(a). Sols

When a colloidal solution appears as
fluid it is referred to as Sol. The sols are generally named
after dispersion medium. For Instance, when the dispersion medium is water, the
sol is known as Hydrosol or Aquosol. When the dispersion medium
is alcohol and benzene, the sols are called Alcosol and Benzosol
respectively.

(b). Gels

When a colloidal has
solid like appearance it is called Gel. The rigidity of gels varies
from substance to substance. Some examples of gels are: Jelly, butter, cheese,
curd.

(IV). CLASSIFICATION BASED ON MOLECULAR SIZE OF
THE DISPERSED PHASE

On the basis of molecular size the
colloids can be divided into three types:

(a).
Multimolecular Colloids

In these colloids, the Individual
particles of the dispersed phase consists of aggregates of atoms or small
molecules having diameter less than 10–7cm. For instance a gold sol consists of particles of various size
containing several atoms. Likewise, a sulphur sol contains particles each
having eight sulphur atoms (S8). In these colloids, the particles are held by weak vanderwaal’s
forces.

(b).
Macro Molecular Colloids:

In these colloids, the particles of
dispersed phase are sufficiently large in size enough to be of colloidal
dimensions. These are called Natural Polymers.Some typical examples are starch, cellulose and proteins. Some
artificial or synthetic polymers are polythene, polystyrene, nylon and
plastics.

(c). Associated Colloids

These colloids behave as normal
electrolytes at low concentrations but behave as colloids at higher
concentration. These associated colloids are also referred as Micelles.
Sodium Stearate behaves as electrolyte in dilute solution but as colloid at
higher concentration. Some other examples of associated colloids are; Soaps,
higher alkyl sulphonates, polythene oxide etc.

(V) CLASSIFICATION BASED
ON THE INTERACTION OR AFFINITY OF TWO PHASES

The colloidal
solutions can also be classified in terms of the attraction between the
dispersed phase and dispersion medium as:

(a). Lyophilic
Colloids

Colloidal solutions in which the
dispersed phase has a great affinity or love for the dispersion medium are
termed as Lyophilic Colloids. Such substances have a tendency to pass
directly into colloidal solution when brought in contact with dispersion
medium. The lyophilic colloids are also called Intrinsic Colloids.

If the dispersion medium is water,
they are called Hydrophilic or Emulsoids. The lyophilic colloids
are generally self-stablised, reversible in nature and are heavily hydrated.
The typical examples of lyophilic colloids are starch, geletin, rubber, protein
etc.

(b). Lyophobic
Colloids

Colloidal solutions in which the
dispersed phase has no affinity or hates the dispersion medium are called Lyophobic
Colloids. These are also referred as Extrinsic Colloids. Such
substances have no tendency to pass into colloidal solution when brought in
contact with dispersion medium. The lyophobic colloids are relatively unstable
and readily tend to change into a suspension. They are irreversible by nature
and are stablised by adding small amount of electrolyte. They are poorly
hydrated.

If the dispersion medium is water, the lyophobic colloids
are called Hydrophobic or Suspensoids. The important typical
instances of lyophobic colloids are sols of metals like Au, Ag, sols of metal
hydroxides like Al(OH)3,
Fe(OH)3 etc and sols of metal sulphides like AS2S3.